This invention relates to the field of industrial process control, and, more particularly to a system for controlling the speed of a DC motor utilized as part of an industrial process control system.
In many industrial applications utilizing DC motors, it is extremely important that the speed of the motor by accurately controlled in order that the output of the industrial process not change over a period of time. For example, the in-line semiconductor wafer processing system marketed by the assignee of the present invention utilizes one or more motor driven direct-drive wafer spin stations which are equipped with a vacuum chuck for the purpose of holding and spinning a wafer at speeds up to approximately 8000 rpm, during which time various chemicals are applied and dispersed as required by the particular process step being performed. It is clear that the speed of the drive motor must be accurately controlled in order for the output of the industrial process to be repeatable and reproducible from run to run.
Systems for controlling the speed of DC motors are not new. In particular, "velocity feedback" techniques are known and utilized for such purposes by those skilled in the art.
One of the drawbacks and limitations inherent in the known techniques for controlling motor speed is that over the life of the control system one or more of the characteristics or parameters of the system may change. For example, changes in friction, temperature, amplifier gain, etc., all of which affect the speed of the motor, can manifest as speed error in excess of 50 to 100 rpm. Since the speed, in the above example, is the speed of a silicon wafer on which various chemicals are being applied and dispersed "on the fly", these speed changes represent an unwanted factor in the chemical "process" being performed. Moreover, they are an annoyance to the user and are a possible caused of "yield" problems. Accordingly, it is clear that such speed changes are highly undesirable.
An optimal speed control system for DC motors typically utilized in the field of industrial process control would possess certain important attributes. In particular, it should be capable of controlling the speed of the motor to a tolerance on the order of .+-.2 rpm. Moreover, it should be relatively inexpensive to implement and should be versatile, i.e., capable of being used with various types of DC motors. And finally, it should be reliable, simple and readily adaptable to the motor.
Prior to the present invention it is believed that the above needs have gone unfulfilled.
It is accordingly a general object of the present invention to overcome the aforementioned limitations associated with existing speed control techniques and to fulfill the needs mentioned by providing a speed control system for a DC motor having characteristics more nearly approaching the optimal attributes noted above.
It is a specific object of the present invention to provide a relatively inexpensive system for controlling and maintaining the speed of a DC motor to an accuracy of about .+-.2 rpm or better.
Other objects will be apparent in the following detailed description and the practice of the invention.